CA1197056A

CA1197056A - Articular prosthesis and its preparation process

Info

Publication number: CA1197056A
Application number: CA000412043A
Authority: CA
Inventors: Gilbert Gaussens; Maryvonne Nicaise; Kieu O. Tran
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1981-09-24
Filing date: 1982-09-23
Publication date: 1985-11-26
Also published as: JPS5865159A; FR2513112A1; DK157059B; NO153914B; DK157059C; US4888019A; DK424382A; EP0076714A1; DE3268301D1; NO823176L; JPH0235579B2; EP0076714B1; FR2513112B1; NO153914C

Abstract

ABSTRACT OF THE DISCLOSURE
A polymer articular prosthesis having a sliding surface, wherein the latter is constituted by a thin layer of said polymer grafted with tetrafluoro-ethylene.
The process for preparing the articular prosthesis comprises coating with a protective varnish the outer surface of a polymer articular prosthesis having a sliding surface, except in the areas constituting this sliding surface irradiating the prosthesis with ionizing rays, contacting the prosthesis with the tetrafluoroethylene vapour for adequate time to bring about tetrafluoroethylene grafting over a limited thickness of the polymer forming the sliding surface and then removing the protective varnish.

Description

~7~S~

ARTICULAR P OSTH S[S ND_ITS PREPARATION PROC-ESS.
BACI~ ~ NJ~ INVENTION.
The present invention relates to an improved polymer articular prosthesis and to its preparation process. More specifically, it relates to a total or partial articular prost-hesis, whose properties have been improved both with respect to the friction coefficients of its sliding surface and with respect to its resistance to deformation.
For some years now, the problems caused by the treatment of patients such as those sufering from arthrosis of the hip have been solved by surgical operations consisting of removing the attacked parts or the joint and replacing them by metallic, ceramic or plastic prostheses in order to obtain friction coe~ficients compatible with a sa~isfactory operation of the joint.
The human joint has in fact a particularly low friction coefficient and the specific properties of the synovia and the cartilage enable the joint to respond to the needs resulting from long-term frictional work under stress.
When it is necessary to use articular prostheses 9 it is not the intention to replace all the mechanisms controlling a healthy joint, bu-t solely to fit new articular surfaces, whilst then assisting the adaptation to the prosthesis of the neuromuscular control, the ligament system, the capsule and the synovia, every effort being made -to preserve their functions.
In order to facilitate this adaptation~ the 7~S6 prostheses are made so as to be able to reproduce as faithfully as possible the natural shapes and the geometry of the joint. Moreover, the choice of the materials used for the prosthesis is very impor~ant, in view of the sliding properties which it must have and the forces exerted on the hinge points constituted by the joints. Thus, the articular slidin~ surfaces must have an easy friction, but must not wear to an exaggerated extent, because it is necessary to prevent the formation of wear debris or fragments which, as a function ~ their grain sizeS could lead to inflammation.
Hitherto, articular prostheses have been made from metal, ceramics or plastics.
Metallic prostheses have the disadvantage o leading to the formation of wear debris~ which is very unsatisfactorily tolerated when the sliding surfaces of the joint are both made Erom metal. Metallic prostheses have in any case been largely abandoned~ due to their defects.
Ceramic prostheses have the advantage of very low friction coefficients when used either in direct contact with the healthy part of a joint, or in contact with a polyethylene or ceramic part. However, they have the disadvantage of being fragile and brittle, which leads to in vitro breaking accidents.
Plastic prostheses are generally used in contact with a metallic part~ which makes it possible to obtain a very low friction coefficient of the jointO
The metal is either a chromi~m - cobalt alloy, or stainless steel and the plastic prosthesis is generally ~'7~ S~

made from high density polyethylene. However, these articular prostheses have the disadvantage of leading to the formation of relatively well tolerated, but unacceptable wear debris, whilst suffering from deformations due to creep,which are prejudicial to the satisfactory functioning of the prosthesisO
BRIEF SUMMARY OF THE INVENTION.
.
The present invention relates to an improved articular prosthesis made from polymerized plastic, which obviates the aforementioned disadvantages.
To thls end, the polymerarticular prosthesis comprises a sliding surface, which is made from a thin layer of said polymer grafted with tetrafluoroethylene~
As a result of the thin tetrafluoroethylene-grafted polymer layer, the friction coefficient of theprosthesis is improved, whilst retaining the -interesting - mechanical properties of the polymer from which it is made.
The process used leads to a local modification of the polyethylene by grafting C2 F4 and to the formation of an ethylene - tetrafluoroethylene copolymer, which makes it possible to improve the friction characteristics and provides better creep resistance. Thus, the simul-taneously applied crosslinking prevents the irreversible sliding of the copolymer chains observed ;n the case of pure polytetrafluoroethylene which, under the action of radiation, degrades and does not crosslink. Thus, the mobility of the polymer chains increased by the presence of fluorine-containing elements does not lead to creep and instea~ facilitates the return of the copolymer to its ~ ~ 9 7 initial structure.
Crosslinked ethylene - polytetrafluoroethylene copolymer (CH2 - CH)n .... (CF2 - CF2) .... (CH CH2)n (CH2 - CH)n .... (CF2 - CF2)n .... (CH CH2)n Non-crosslinked polytetrafluoroeth~ene polymer ..... (CF2 - ~F2 ..... (CF2 - CF2)... ~
In general, the thickness of the grafked polymer layer is approximately 003 to 0.5S~lm.
According to a preferred embodiment of the invention, the prosthesis is made from a crosslinked polymer, which leads to a further improvement of its mechanical properties. In this case9 it advantageously comprises a part constituted by polymer crosslinked to a higher degree than the remainder of the prosthesis.
The location of the more highly crosslinked part is chosen as a function of the mechanical stresses undergone by the prosthesis in order to correspond to the l~cation where the compressive stresses are highest.
Thus, the creep resistance of the prosthesis is improved by creating an area of greater hardness at the point where the stresses are highest. As a result of the special prosthesis structure according to the invention, i.e. the presence of areas of polymer crosslinked to different degrees and a sliding surface fo~med from tetrafluoroethylene-grafted polymer, it is possible to obtain appropriate hardnesses and in particular avoid having excessive hardness on theslid;ng surface, which would lead to the formation of ~brasive ~ ~ ~ 7 debris causing greater wear.
In order to obtain good friction coefficients~
it is advantageous for the degree of tetrafluoroethylene grafting of th~ layer of limited thickness forming the sliding surface to be 0.3 to 2.5 mg of tetrafluoroethylene per cm of surface.
It is pointed that this degree of grafting corresponds to the formula P /s in which P represents the weight quantity of tetrafluoroethylene in the prosthesis and s represents the outer surface of the grafted polymer layer constituting the sliding surfaceO
According to the invention, the polymer constituting the prosthesis can be chosen from the group including polyolefins such as polyethylene and polypropylenè, polystyrene, polyacrylates, polyvinylchloride, polyamides and-polyesters.
The choice of polymer depends more particularly on the joint which the prosthesis is to replace and is made whilst taking account of the mechanical properties of the polymer is order to obtain the desired mechanical characteristics of the joint to be replaced (knee, shoulder, ankle, fingers~ etc). Preferably and in particular in the case of the hip prosthesis, the polymer is polyethylene.
The invention also relates to a process for the preparation of a prosthesis having the aforementioned characteristics and which comprises:
a) coating with a protective varnish, which is impermeable and inert with respect to tetrafluoroethylene 9 the outer surface of a polymer articular prosthesis having a sliding s~

surace except in the area or areas for~ing the said sliding surface, b) irradiating with ionizing rays the thus coated prosthesis, c) bringing the thus irradiated prosthesis into contact with tetrafluoroethylene vapour for an adequate period to bring about tetrafluoroethylene grafting on a limited thickness of the polymer forming the sliding surface, and d) eliminating the protective varnish.
When working in this way and carrying out irradiation under appropriate conditions, it is possible to obtain on the one hand a crosslinking of the polymer constituting the prosthesis and on the other the grafting of the tetrafluoroethylene solely on that surfa~e of the prosthesis which is not protected by the varnish.
The varnish serves merely to protect the prosthesis, i.e. to render it waterproof in order to prevent any contact with the monomer in the vapour state and consequently prevent the grafting of the rnonomer to the areas of the prosthesis which are protected.
This varnish must be impermeable and iner-t to tetrafluoroethylene, have a good wettability with respect to the polymer, have a good stability, have a limited susceptibility to tearing9 remain flexible and 25 have an adequate resistance to ioni~ing radiation. Moreover, it must have an adequate adhesion in order to effectively protect the prosthesis and must be easy to eliminate then at the end of the operation, preferably by detaching it.
Finally, it is preferable that it retains its mechanical properties at low temperatures, e.g. at -100C.

~ '7~ 5~

Thus, in order to bring the ooated prosthesis into contact with the gaseous monomer, there is generally a transfer of the gaseous monomer by cooling the conta mer in which the prosthesis elements to be grafted are located.
As varnishes which can be used, reference is made to the varnishes based on vinyl resins, such as the product marketed under ~e trade name Nucletex and which corresponds to the following formulation:
- vinyli-te resin VYHH 8 parts by weight (87% vinyl chloride, 13% vinyl acetate) - vinylite resin VYNS 18 parts by weight (90% vinyl chlorideg 10% vinyl acetate) ~ paraflex (polyester) 8 parts by weight - dioctyl phthalate 6 parts by weight - solvent (methyl ethyl ketone) until dissolved.
This varnish is applied to the prosthesis by conventional processes 9 e.g. by means of a brush.
After drying, the thus protected prosthesis is irradiated with ionizing radiation, which leads both to the crosslinking of the polymer and to the production o free radicals forming the active sites for the grafting.
This radiation is carried out in the absence of oxygen9 e.g. under vacuum or in an inert gas atmosphere 9 e.g.
a nitrogen atmosphere. The ionizing radiation which can be used are Y rays, ultraviolet rays or electron beams. Preferably, the irradiation is carried out by means of an electron beam having an energy of 2 5 to 3 MeV with a dose of 10 to 15 Mrad.
After irradiat-ion, the prosthesis is brought ~ 5~

into contact with the tetrafluoroethylene vapour in order to graft it to the polymer in the prosthesis area or areas not protected by the varnish. This contacting operation is carried out at a temperatllre and for a time chosen as a function of the degree of grafting which it is desired to obtain. This degree of grafting can be controlled by acting on the energy and irradiation dose applied, on the pressure of the monomer, on the temperature and on the contact time with the monomer.
In order to obtain a degree of grafting of 0.3 to 2.5 mg/cm2, the prosthesis is contacted with the tetrafluoroethylene for 50 to 70 hours at a tetrafluoro-ethylene pressure of 1.2 to 1 6 bar.
The degree of crosslinking of the polymer forming the prosthesis can be regulated by acting on the energy of the ioni~ing radiation beam5 on the irradiation dose and on the orientation of the prosthesis with respect to the beam.
According to a variant of the process according to the invention, the irradiation of the prosthesis is carried out in two stages, in order to bring about in the first stage the crosslinking of th2 polymer forming the prosthesis and in order to bring about in the second stage the pre-irradiation necessary for grafting.
In this case, after applying the varnish to the parts of the prosthesis to be protected, the first stage is preferably carried out by means of ~ radiation and the second stage by means of an electron beam.
Howe~er, it is possible to apply the varnish ~8--~ ~ 7 ~ 56 between the two irradiation stages in which case the process comprises:
a) irradiating a polymer articular prosthesis having a sliding surface by means of ionizing radiation in order to crosslink the polymer, b) coating with a protective varnish which is impermeable and inert with respect to tetxafluoroethylene the outer surface of the crosslinked polymer articular prosthesis, with the exception of the area or areas forming the sliding surface, cj irradiating the thus pro~ected prosthesis by means of ionizing rays, d) bringing the thus irradiated prosthesis into contact with the tetrafluoroethylene vapour for a period adequate to obtain the grafting of the tetrafluoroethylene to a limited thickness of the polymer forming the sliding surface, and e) eliminating the protective varnish.
In the same way as indicated hereinbefore, the crosslinking stage is preferably carried out by irradll~.ing then with ~ rays, whilst the irradiation stage of the varnish-protected prosthesis is carried out by means of an electron beam.
In order to obtain in the prosthesis a part 25 formed from crosslinked polymer having a higher degree of crosslinking than the remainder of the prosthesis, it is possib~e to use ~nventional processes, e.g. choosing the energy of the radiation treatment so as to obtain dif:Eerent degrees of crosslinklng~ Preferably, in order to achieve this result~ the irradiation is carried out _9~

'7~56 in such a way that the prosthesis rece;ves the ionizing radiation under two different orientations.
In this case, the irradiation of the prosthesis can be carried out in two stages and the prosthesis position relative to the radiation beam can be modified between the two stages, ;n such a way that one area of the prosthesis receives two irradiation d~es, whilst the remainder thereof only receives a single irradiation dbse. Thus, a higher degree of crosslinking is obtained in the area having received two doses. The position of this area is chosen in such a way that it cor-responds to the orientation axis of the joint supporting the highest pressures and stresses~
BRIEF DESCRIPTION OF THE DRAWINGS.
The invention is described in greater detail hereinafter relative to non limitative examples and the attached drawings, wherein show:
Fig 1 the device used for testing the deformation resistance of articular prostheses according to the invention.
Figs 2 and 3 the kinematics of the test device.
Fig 4 deformations observed on the cotyloid of khe prosthesis of example 1.
Fig 5 the deformations observed on the cotyloid of the prosthesis of example 2.

This example relates to the treatment of a polyethylene hip prosthesis, whose shape corresponds to the part of the hip bone including the cotyloid.
Firstly, a varnish constituted by the product ~9 7 ~ S~

Nucletex is applied by means of a brush to the outer surface of the pros~hesis, excep-t the inner cavity of the cotyloid. The thus coated prosthesis then undergoes ~ irradiation in vacuo by means of a cobalt 60 source under the following conditions:
- dose rate : 0.27 Mrad/h dose : 15 Mrad~
The prosthesis then undergoes a ~urther irradiation in vacuo by means of an electron beam having an energy of 3 MeV, a beam intensity of 400 ~A and a dose of 13 Mrad. Following irradiation, tetrafluoroethylene is introduced into the vacuum enclosure containing the prosthesis at a pressure of 1.5 bar and a temp~rature of 15C and the prosthesis is left in contact with the tetrafluoroethylene for 50 hours, which makes i~ possible to graft the tetrafluoroethylene to the unprotected parts of the prosthesis. Following this grafting stage3 the protective varnish coating is removed by disengaging it from the prosthesis. The d~gree of grafting is then deter-~0 mined and this corresponds to formula m with Pm =P~-Pi in which pf represents the weight of the proskhesis after grafting, Pi the weight o the prosthesis before grafting and S the surface of the prosthesis not protected by the varnish. Under these conditions, the degree of grafting obtained is 0.318 mg of tetrafluoroethylene per cm of unprotected surface.
The thus obtained prosthesis then undergoes wear tests carried out by means of the device shown ;n Fig lo This device comprises a geared motor 1, to which is vertically fixed a trolley 2 moving in accordance with an axis 3, a ~ 7~

cam 3 integral with the geared motor l and in perrnanent contact with a fixed roller 4, a crank means 6 driven by the geared mo~or 1 and on which is installed a cardan joint 5 receiving the base of a femur 7, a second trolley 8 perpendicular to the first and moving in a vertical plane supporting the prosthesis 9 to be tested and the ]oad 10. The device also comprises a distilled water supply, used for lubrica~ing the parts in friction~
In operation9 the geared motor 1 drives the crank 6, whose circular movement is deteriorated by cam 3 in contact with roller 4, so that the circular movement of crank 6 is accompanied by an alternating displacement of the centre of rotation of the crank in the actlon plane of the trolley. Therefore, the displacement of the crank is modified and approximately assumes an elliptical shape and the length difference between the major axis and the minor axis of the ellipse produces, at the head of the femur, two displacements of the vertical trolley 8 supporting prosthesis 9. One revolution of cam 3

2~ ensuring the d;splacement of femur 7 and prosthesis 9 corresponds to one cycle. Fig 2 shows the movement described by the head of the femur in the cotyloid of prosthesis 9 during one cycle. This movement is broken down in the manner indicated hereinafcer.
An angular displacement of the femur 7 from the back to the front is obtained by describing, relative to the vertical and in the walking direction, firstly an angle of 25 and then an angle of 35, the limit between these two angles being the vertical plane 0 perpendicular to the step in which passes the axis of ~ 7~ S6 symmetry A of the skeleton. This lateral displacementtowards the outside is accompanied by a displacement by a maximum angular value of 15 in the same vertical plane. To return to the initial position~ there is an inward lateral displacement of 10. In addition to the two movements described hereinbefore, during one cycle, the femur pivots coaxially at the diaphysis by an angle of ~6. All the alternating and rotary movements described hereinbefore during a cycle at the knee, determines the asymmetrical figure shown in Fig 3, whose limits are a function of the allowed angular displacements.
This arrangement ensures a total scan of the cotyloid by the head of the femur under conditions very similar to those encountered in vivo for the most frequently encountered positions of the lower member at the knee (forward movement, bac~ard movement5 direction change, etc).
For the purpose of the present tests, the load 10 applied to the mobile means supporting the prosthesis is 100 daN and the geared motor is operated at a speed of 25 cycles per minute. At the end of 500,000 cycles, the profile of the cotyloid is examined and ~ geometry is compared with that of an untreated polyethylene cotyloid.
The results obtained are illustrated in Fig 4 which shows the deformations observed on the coLyloid treated by the process of the invention (4a, 4b9 4c, 4d) and the deformations observed on an iflentical untreated cotyloid (4e).

~97~6 Figs 4a, 4b, 4c, 4d are sections of the cotyloid respectively in accordance with pl.anes perpendicular to the base of the hem-Lsphere deined by the inner cavlty of the cotyloid and passing through the centre of said hemisphere, said planes being displaced by 45 relative to one another.
It is apparent that the cotyloid treated by the process of the invention has few deformations compared with the untreated cotyloid, whose section is made in accordance with the same plane as that of Fig 4a.
Example 2 A polyethylene prosthesis identical to that of example 1 is used and the same varnish is applied to its outer surface, with the exception of the inner cavity of the cotyloid. The thus coated prosthesis undergoes irradiation in vacuo by means of an electron beam with an energy of 3 MeV, a beam intensity of 400 ~A and a dose of 13 Mrad, by orienting the prosthesis in such a way that the beam is perpendicular to the concavity of the spherical part~
The irradiated prosthesis is then brought into contact with the tetrafluoroethylene under a ~ressure of 1.5 bar, at a temperature of 15C and for 50 hours. Under these conditions, the degree of grafting is 0.462 mg of tetrafluoroethylene per cm2.
The thus treated prosthesis is subject to the wear tests performed under the same conditions as in example 1. The results obtained are given in Fig 5.

.97~S6 In Fig 5~ references 5a, 5b, 5c and 5d relate to sections made under the same conditions as sections 4a, 4b, 4c and 4d of Fig 4 and 5e relates to a section of an untreated cotyloid made in the same plane as section 5a.
It is apparent that the deformations of the treated cotyloid are much less pronounced than those of the untreated cotyloid. Furthermore, compared with the prosthesis obtained in example 1, the results are better, the de~ormations being less pronounced and more regular with respect to the centre of the cavity.
It would seem that in the case of example 1 where the crosslinking is performed by irradiation before grafting, a higher crosslinking of ~he polymer is obtained and consequently a lower degree of tetrafluoroethylene grating, because the tetrafluoro-ethylene penetr~ting the polymer by diffusion does so less well as a result of the crosslinking.
In addition, due to the higher degree of crosslinking and the lower degree of grafting, the hardness of the prosthesis sliding surface is higher and the debris formed doubtless too abrasive, which leads to higher wear to the prosthesis.
Example_3 .

This example relates to the grafting of the sliding surface of a polyethylene prosthesis identical to that of example 1.
Crosslinking and grafting are carried out simultaneoucly by using the experimental irradiation conditions of example 2 after protecting the outer surface ~ ~'7~ ~ ~

of the prosthesis, with the exception of the inner cavity of the cotyloid, by means of the same protective varnish as in example l.
Following irradiation, the prosthesis is brought into contact with the tetrafluoroethylene ak a pressure o 1.5 bar, a temperature of 15C and a time of 65 hours~ followed by the removal of the varnish.
Under these conditions, the degree of grafting obtained îs 1.32 mg of tetrafluoroethylene per cm2~ The thus obtained prosthesis undergoes an examina~ion by electron spectroscopy, which makes it possible to locate the fluorine atoms~ This reveals that ~hese atoms are located at a depth up to 3000 ~, which shows that the grafting has taken place on a polymer layer of limited thickness.

A polyethylene prosthesis identical to that of example 1 is used and it undergoes the coating and irradiation operations under the same conditions as in exam~le 2.
After irradiatio~ the prosthesis is brought into contact with the tetrafluoroethylene at a pressure of 1.5 bar, a temperature of 20C and a time of 65 hours.
The degree of grafting obtained is 2 mg of tetrafluoro ethylene per cm ~ The prosthesis is then examined by electron spectroscopy to locate the fluorine atoms.
In this case~ the fluorine atoms are at a depth of 5000 R~ Thus, this grafting method makes it possible to graft the tetrafluoroethylene over a limited thickness of the unprotected surface of the prosthesis.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A polymer articular prothesis having a sliding surface, wherein the latter is constituted by a thin layer of said polymer grafted with tetrafluoroethylene.

2. A prothesis according to claim 1, wherein the polymer forming the prosthesis is crosslinked.

3. A prosthesis according to claim 2, wherein part of the prosthesis is formed from a polymer crosslinked to a higher degree than the remainder of the prosthesis.

4. A prosthesis according to claim 1, wherein the polymer is polyethylene.

5. A prosthesis according to claim 1, wherein the polymer is chosen from the group including polyolefins, polystyrene, polyacrylates, polyvinylchloride, polyamides and polyesters.

6. A prosthesis according to claim 1, wherein the degree of tetrafluoroethylene grafting of the thin layer forming the sliding surface is 0.3 to 2,5 mg of tetrafluore-ethylene per cm2 of surface.

7. A process for the preparation of an articular prosthesis according to claim 1, wherein it comprises:
a) coating with a protective varnish, which is impermeable and inert with respect to tetrafluoroethylene, the outer surface of a polymer articular prosthesis having a sliding surface except in the area or areas forming the said sliding surface, b) irradiating with ionizing rays the thus coated prosthesis, c) bringing the thus irradiated prosthesis into contact with tetrafluoroethylene vapour for an adequate period to bring about tetrafluoroethylene grafting on a limited thickness of the polymer forming the sliding surface, and d) eliminating the protective varnish.

8. A process according to claim 7, wherein irradiation is performed by means of an electron beam.

9. A process according to claim 7, wherein irradiation is performed in two stages, the first stage being carried out by means of .gamma. radiation and the second stage by means of an electron beam.

10. A process for the preparation of an articular prosthesis according to claim 1 wherein it comprises:
a) irradiating a polymer articular prosthesis having a sliding surface by means of ionizing radiation in order to crosslink the polymer, b) coating with a protective varnish which is impermeable and inert with respect to tetrafluoroethylene the outer surface of the crosslinked polymer articular prosthesis, with the exception of the area or areas forming the sliding surface, c) irradiating the thus protected prosthesis by means of ionizing rays, d) bringing the thus irradiated prosthesis into contact with the tetrafluoroethylene vapour for a period adequate to obtain the grafting of the tetrafluoroethylene to a limited thickness of the polymer forming the sliding surface, and e) eliminating the protective varnish.

11. A process according to claim 10, wherein crosslinking is carried out by irradiation by means of .gamma. rays.

12. A process according to either of the claims 10 and 11, wherein irradiation of the varnish-protected prosthesis is carried out by means of an electron beam.

13. A process according to claim 7, wherein irradiation is carried out in such a way that the prosthesis receives the radiation under two different orientations, in order to obtain a higher degree of polymer crosslinking in part of the prosthesis.